1. Field of the Invention
The present invention relates to a transconductor and, more particularly, to a method and circuit for varying transconductance, and a filter and an amplifier using the same.
2. Description of the Related Art
Recently, an optical disk reproducing apparatus for reproducing a signal from a disk, such as a compact disk (CD), a digital video disk (DVD), etc., has shown a tendency toward a high reproducing speed. In the optical disk reproducing apparatus of a low reproducing speed, such as normal, double, quadruple or octuple speed, the frequency of an input signal picked up from the disk is low. Accordingly, there is no need to use an equalizer. However, in the optical disk reproducing apparatus of a high reproducing speed, the frequency characteristic is very important. In particular, as the reproducing speed increases, the frequency of the input signal becomes high. Since a level of the input signal picked up from the disk becomes low at higher frequencies, it is necessary to install the equalizer to boost the high frequency signal. In the optical disk reproducing apparatus of a DVD type, a signal received at a pickup has different levels according to frequency and these levels depend on the kind of disk and the kind of pickup. In order to appropriately process the input signal, a high performance equalizer should be used.
Since there are many kinds of pickups and disks used in the optical disk reproducing apparatus, the frequency characteristic of the equalizer, i.e., the frequency characteristic of a filter utilized in the equalizer should be set according to the kind of disk and the kind of pickup. However, if each frequency characteristic corresponding to the kind of disk and the kind of pickup used in the optical disk reproducing apparatus must be provided by additional filters, and if one filter is selectively used as the need arises, the construction of. the equalizer is complicated and the stability thereof is degraded.
Meanwhile, a transconductor generates an output current which is proportional to an input signal voltage. Assuming that the input signal voltage is Vin and the output current is i, the output current i=gm.times.Vin (where gm is a transconductance of the transconductor). If a capacitor is connected to the output of the transconductor, the characteristic of an output signal voltage Vo for the input signal voltage Vin can be expressed as follows: Vo/Vin=gm.times.(1/sC) (where C is a capacitance of the capacitor and s is the Laplace variable). The combination of the transconductor and the capacitor can be used in a filter. The characteristic of such a filter is represented by gm.times.C. The transconductor may also be used in an amplifier.
If the filter using the combination of the transconductor and the capacitor is utilized in the equalizer of the optical disk reproducing apparatus, the frequency characteristic may be varied according to the transconductance. Therefore, the frequency characteristic corresponding to the kind of disk and the kind of pickup can be set by varying the transconductance of the filter In other words, if only a transconductor device having different output transconductances is used in the filter then different frequency characteristics can be provided without providing several separate filters.
If the transconductance varies whenever the disk and the pickup changes, the frequency characteristic of the filter can be variably set even if only one set of filter components is used. To variably set the frequency characteristic of the filter simply and stably, a technique is needed for simply and stably varying the transconductance. Such a technique would also be applicable for setting a variable gain of the amplifier.
As described above, there is need for an apparatus and method for variably setting the frequency characteristic of a filter or the gain of an amplifier simply and stably. To this end, the present invention relates to a technique for simply and stably varying the transconductance of the transconductor used in an amplifier and a filter.